Microwave preheat press assembly

ABSTRACT

The present invention is a microwave preheat press. The microwave preheat press includes a first and a second press section which define a press passage therebetween. A compression belt movable along the press passage is contiguous with one of the press sections. The press passage is configured such that it has an initial press section leading into a final press section. A microwave generator is in communication with a microwave waveguide applicator structure assembly such that microwave energy is created and transmitted through the microwave waveguide assembly into the initial compression section.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.10/265,041, filed Oct. 3, 2002, the benefit of which is hereby claimedunder 35 U.S.C. § 120.

FIELD OF THE INVENTION

The present invention relates to systems and methods of formingcomposite wood products and more specifically to systems and methods offorming composite wood products using a microwave preheat pressassembly.

BACKGROUND OF THE INVENTION

Many processes are known in the art for forming a composite wood elementproduct from a composite mat assembly of resin coated wood elements. Forexample, steam and microwave energy have been used in conjunction with avariety of press assemblies to heat the mat assembly to about the curetemperature of the resin and compress the mat assembly to a desiredfinal dimension. However, the designs of the prior press assemblies arerelatively inefficient.

Steam injection heating sources unduly limit the size of the compositewood element products that can be formed. The heat carried by the steammust naturally conduct into the central portion of the product to raisethe product to the cure temperature. Natural flow through a wood productis limited to certain thicknesses for certain products. Products over 5inches thick can be very difficult to evenly heat. Further, steaminjection presses are fixed in length and are not continuous inoperation, limiting the length of the product. Thus, steam injectionheating does not allow the production of relatively larger wood elementproducts. By limiting the size of product that can be produced from agiven press assembly the press assemblies ultimate utility is limited.

Traditional microwave heating systems have helped solve some of the heattransfer problems. However, the design of these microwave heatingsystems creates another problem. More specifically, current designs haveno balance between the time at which a mat assembly reaches the curetemperature and the time in which the same mat assembly reaches a fullycompressed state. The result is either a mat assembly where the resin iscured before reaching final compression or a failure to bring the mat tothe optimal temperature at all. The percentage of the energy that can bedelivered by preheat is thus limited. In both cases an undesirableresult is attained.

Press assemblies using microwave applicators directing microwave energyinto the sides of the mat assemblies have been developed in an attemptto address these issues. However, the side application systems havetheir limits as well. Placing a microwave waveguide assembly within apress limits the type of press assemblies that can be used. Morespecifically, many press assemblies are designed such that it isphysically impossible to place a waveguide assembly at the side of thepress. Consequently, press assemblies that may otherwise be highlydesirable for other reasons may be prevented from being used by theirdesign.

Examples of various microwave curing systems are U.S. Pat. No. 5,228,947issued Jul. 20, 1993, U.S. Pat. No. 6,290,809 B1, issued Sep. 18, 2001,U.S. Pat. No. 6,242,726 B1, issued Jun. 5, 2001, U.S. Pat. No.4,020,311, issued Apr. 26, 1977, U.S. Pat. No. 4,456,498, issued Jun.26, 1984 and U.S. Pat. No. 6,176,951 B1, issued Jan. 23, 2001. All ofwhich are incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention relates to an improved system and method forapplying microwave energy for curing a composite wood product within apress assembly.

The present invention is a microwave preheat press. The microwavepreheat press includes a first and a second press section which define apress passage therebetween. A compression belt movable along the presspassage is contiguous with one of the press sections. The press passageis configured such that it has an initial press section leading into afinal press section. A microwave generator is in communication with amicrowave waveguide assembly such that microwave energy is created andtransmitted through the microwave waveguide assembly into the initialcompression section.

The present invention also includes an improved method of forming aconsolidated composite wood product from a mat assembly formed ofdiscrete wood elements coated with a resin having an effective curetemperature. The method includes introducing the mat assembly into aninitial compression section of a press; directing an amount of microwaveenergy into the mat assembly while the mat assembly is within theinitial compression section; compressing the mat assembly into a finalcompression state. The amount of microwave energy is sufficient to bringthe mat assembly to, or very close to, the cure temperature of the resinas the mat assembly reaches a final compression state or shortlythereafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred and alternative embodiments of the present invention aredescribed in detail below with reference to the following drawings.

FIG. 1 is a side view of the press assembly with microwave pre-heataccording to the present invention;

FIG. 2 is side view of another embodiment of the present invention;

FIG. 3 is a top schematic view of various aspects depicted in FIG. 1;

FIG. 4 is perspective view of an embodiment of the present invention;

FIG. 5 is perspective view of another embodiment of the presentinvention;

FIG. 6 is perspective view of yet another embodiment of the presentinvention; and,

FIG. 7 is perspective view of additional embodiment of the presentinvention;

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a system and method for forming compositewood element products using a microwave preheat. By way of overview, andwith references to FIG. 1, one presently preferred embodiment includes amicrowave preheat press 10. The microwave preheat press 10 includes afirst press section 11 and a second press section 13 that define a presspassage 15 therebetween. The press passage 15 is configured to define apreheat section 19, an initial compression section 21 and a finalcompression section 22. A microwave impervious compression belt 12, 14is contiguous each press section 11, 13, respectively. A microwavegenerator 54 is used to generate microwave energy. A microwave waveguideapplicator structure assembly 40 is adjacent the initial compressionsection 21 and is configured to direct microwave energy from themicrowave generator 54 into the initial compression section 21. Specificdetails of the microwave preheat press 10 are described in moreparticularity below.

Referring now to FIG. 1, the present invention is preferably used toform composite wood element products 28 from a mat assembly 26. The matassembly 26 preferably comprises an arrangement of wood elements, suchas strands, chips, strips, veneer or particles coated with a resinhaving a cure temperature, and that can be subjected simultaneously topressure and microwave energy to form a composite wood element product28. Suitable, non-limiting examples of such composite wood elementproducts 28 include particleboard, fiberboard, waferboard, plywood,oriented strand board, laminated veneer lumber, parallel strand lumber,and laminated beams. The dimensions of the composite wood elementproducts 28 formed by the present invention will be a matter of choicewithin the discretion of those skilled in the art.

The resin used to coat the wood elements of the mat assembly 26 ispreferably an alkaline phenolic resin. However, any adhesive whose rateof cure is accelerated by the application of heat may be used with thepresent invention. Non-limiting examples of such resins are watersoluble and non-water-soluble alkaline and acidic phenolic resins,resorcinol-formaldehyde resins, urea-formaldehyde resins, and isocyanateresins. The resins may be applied to the wood elements in any desiredamount, or as necessary to form the specific compressed composite woodelement product.

Continuous belt-type presses employable with the present invention areknown in the art. As such, a detailed description of their structure isnot necessary for the understanding of this invention. The continuouspress used in the present invention generally includes a first presssection 11 and a second press section 13. The press sections 11, 13 arespaced apart to form a press passage 15 therebetween through which themat assembly 26 may be passed. Suitable examples of press-types useablewith the present invention are continuous presses made by Metso orDieffenbacher.

The first and second press sections 11, 13 define an entrance section20. The entrance section 20 is suitably arranged to receive the matassembly 26. Part of the entrance section 20 is an initial compressionsection 21. The initial compression section 21 is considered that partof the press where the mat assembly 26 is being compressed, but has notyet been fully compressed. The final compression section 22 is the presssection wherein the mat assembly 26 is at a full compressed state.

The compression belts 12, 14 are preferably stainless steel belts.However, other types of metal belts such as molybdenum belts may beused. Further, the compression belts 12, 14 may optionally be coatedwith a plastic material (not shown). For example, stainless steel pressbelts coated with a film of polytetrafluoroethylene may be used.Alternatively, compression belts 12, 14 having a metal coating on anon-metallic support may be used.

The microwave waveguide applicator structure assembly 40 is preferablypositioned adjacent one of the compression belts 12, 14 so that theleading end 41 is substantially in contact with their respective belts12 and 14. Contact is not required within the scope of the invention butit is preferable to be as close as practical. Separations of many inchesare acceptable. The microwave waveguide applicator structure assembly 40is configured to direct microwave energy from the microwave generator 54through window 44 and into mat assembly 26 while the mat assembly 26 iswithin the initial compression section 21.

As best illustrated in FIGS. 1 and 2, suitable chokes 45, 47 may bepositioned at the leading ends 39, 41 to inhibit arcing between themicrowave waveguide assembly applicator structure 38, 40 and theiradjacent belt 12, 14, respectively. The structure of such chokes 45, 47is well known in the art and may be of any suitable form. Also, toinsure proper positioning, the microwave waveguide applicator structureassembly 40 may be provided with any commonly used wear plate 35, 37, orsimilar device. Further, suitable windows 42, 44 may also be used toallow the entrance of microwave energy while providing support for themat assembly 26. Also, in the regions around the microwave waveguideapplicator structure assembly 40, suitable microwave type structures 52,such as dielectric spacers (FIG. 3) may be used to help control themicrowave energy.

Optionally, microwave heating in the press may be supplemented byheating the press belts conventionally, e.g., by heating platens (notshown) over which the belts may run, using, for example, steam or oil.The platen heating can both increase the final rate of cure and preventthe press surfaces from prematurely cooling the pressed assembly. Meansfor implementing such conventional press heating are well known in theart.

The number and overall arrangement of the microwave waveguide applicatorstructure assembly 40 employed with the present invention is notintended to limit the present invention. More specifically, FIG. 1discloses only one microwave waveguide applicator structure assembly 40located on the bottom of the microwave preheat press 10. However, themicrowave waveguide applicator structure assembly 40 could just aseasily be placed on the top. Alternatively, two or more independentmicrowave generators 54 may be used with separate waveguide applicatorstructure assemblies 38 b, 40 b. These multiple sources may be arrangedon the same or opposite sides of the press passage 15. FIG. 2, depict anarrangement where a single microwave waveguide applicator structureassembly 38 b, 40 b is located on both the top and bottom of themicrowave preheat press 10. FIGS. 6 and 7 depict another possiblearrangement, where multiple microwave waveguide assembles 38 c, 40 c, 38d and 40 d are employed on a single side of the microwave preheat press10. Additionally, combinations of these embodiments (not shown) alsoconsidered within the scope of this invention.

As depicted in FIG. 3, it is preferable that when multiple points ofapplication of the microwave energy are used that the points bestaggered in the direction of 50 of the mat 26 entering press 10 bywaveguide spacing 56. In a preferred embodiment, the waveguide spacing56 is chosen to yield a substantially uniform heating pattern 64 a, 64b, and 64 c is achieved. However, the amount of waveguide spacing 56 isvariable and may be determined by those skilled in the art. Thewaveguides 38 and 40 and the windows 42 and 44 may also be laterallyoffset by a waveguide offset 58, as best seen in FIG. 3. It will beappreciated that both the waveguide spacing 56 and the waveguide offset58 may be selected to achieve a more even heating pattern 64.

To this end, the waveguide offset 58 is preferably equal to about ½ thewaveguide width. In this manner, the peaks of energy from one window 42are aligned with the valleys of the energy peaks applied through theother window 44 and vice versa thereby more uniformly apply microwaveenergy across the lay-up.

In a presently preferred embodiment, a microwave generator 54 producingmicrowave energy at a frequency of 915 MHz is preferred. However, othermicrowave frequencies are considered within the scope of this invention.For example, it has been found that frequencies as low as about 95 MHzcan be employed. The upper frequency limit is not critical and is set bypractical considerations, since there is a direct relationship betweenthe optimum size of a waveguide and the frequency of the electromagneticwave that can be conducted through it. Accordingly, any microwavefrequency ranging from at least about 95 MHz is considered within thescope of this invention.

Commercially available microwave generators 54 which may be effectivelyemployed in the practice of the invention include, inter alia, CoberElectronics and Microdry Magnetrons having a power output of 75–100 kWand operating at 915 MHz, or klystrons which are available at a varietyof power outputs and frequencies.

The amount of microwave energy applied to the mat assembly 26 isvariable and will depend upon various factors such as the nature of thewood elements and the properties of the resin coating. Regardless, theamount of microwave energy applied to the mat assembly is preferably anamount sufficient to bring the mat assembly 26 to a temperature at leastequal to the cure temperature of the resin while the mat assembly isstill within the initial compression section 21. Preferably, the matassembly 26 is brought up to at least the cure temperature at the timethe mat assembly enters the final compression section 22. Alternatively,the mat assembly 26 may be brought to above the cure temperature afterthe mat assembly 26 enters the final compression section 22. Again,alternatively the mat may be brought to a temperature less than the curetemperature and the hot platens of the press will supply the finalenergy needed to cure the assembly. For some products this is preferredand it is within the scope of the invention. These results areaccomplished by controlling several variables.

One variable that is controlled to achieve the desired heating of themat assembly 26 is by moving the mat assembly 26 past the microwavesources at a suitable linear speed. One skilled in the art may determinelinear speeds. By way of non-limiting example, when usingphenol-formaldehyde resins, the linear rate may be controlled so thetemperature for mat assembly 26 may range from about 100° C. to about170° C. In a preferable arrangement the temperature may be within arange from about 110° C. to about 150° C. More preferably the matassemblies 26 are heated to a range from about 115° C. to about 120° C.In applications where finishing the curing is best done in the presstemperature ranges from about 80 degrees C. to about 98 degrees C. maybe preferred.

The linear feed rate at which the mat assembly 26 enters the microwavepreheat press 10 is variable and is dependent upon the nature of thecomposite wood element product 28 being formed. In a presently preferredembodiment, a linear feed rate within the microwave preheat press 10will range from about 0.5 feet per minute to about 150 feet per minute.In a particular embodiment, the linear feed rate is from about 1 toabout 30 feet per minute. However, the present invention is not limitedby the linear feed rate. As such, feed rates above and below thepreferred feed rates are considered within the scope of this invention.

Another variable that is controlled to achieve the desired heating ofthe mat assembly 26 is the control of the microwave waveform. It ispreferable for the microwaves to be propagated in the waveguide assembly40 in a TE_(N0) mode, where N is any integer. In a particular embodimentthe N is equal to 1, yielding a TE₁₀ mode. However, a TE_(N0) mode whereN is any integer greater than one is also within the scope of thisinvention. Additionally, a waveguide assembly 40 producing a waveformhaving a TE_(0N) mode is also considered within the scope of thisinvention. In this particular embodiment, the N is also equal to 1, orany integer greater than 1.

Referring now to FIGS. 4–7, the microwave waveguide applicator structureassembly 40 is also preferably configured such that the electric vectors65, 65 a, 65 b and 65 c are oriented substantially perpendicular todirection of travel 50 of the mat assembly 26. As will be understood bythose having skill in the art, there are two planes in which a vectormay be perpendicular to a third plane. As such, FIG. 5 and 7 depictvarious orientations for the electric vectors 65.

While the preferred embodiment of the invention has been illustrated anddescribed, as noted above, many changes can be made without departingfrom the spirit and scope of the invention. Accordingly, the scope ofthe invention is not limited by the disclosure of the preferredembodiment. Instead, the invention should be determined entirely byreference to the claims that follow.

1. A microwave preheat press comprising: a first and a second press section defining therebetween a press passage; a compression belt moveable along said press passage contiguous with one of said press sections; the press passage having an initial compression section and a final compression section; a microwave generator; a microwave waveguide applicator structure assembly adjacent the initial compression section, disposed between a fibrous mat and the compression belt, the microwave waveguide applicator structure assembly being configured to direct microwave energy from the generator into the initial compression section; and a choke configured to inhibit arcing between the microwave waveguide assembly and the compression belt.
 2. The press of claim 1, wherein the compression belt is constructed from microwave impervious material.
 3. The press of claim 2, wherein the microwave impervious material is at least one of a stainless steel, molybdenum, polytetrafluoroethylene coated stainless steel, and polytetrafluoroethylene coated molybdenum.
 4. The press of claim 1, wherein the microwave generator forms microwaves having a frequency of at least 95 MHz.
 5. The press of claim 1, wherein the microwave waveguide assembly is configured to produce a TE_(N0) mode waveform.
 6. The press of claim 5, wherein N in the TE_(N0) wave mode is equal to
 1. 7. The press of claim 5, wherein the N in the TE_(N0) wave mode is an integer greater than
 1. 8. The press of claim 1, wherein the microwave waveguide assembly is configured to produce a TE_(0N) mode waveform.
 9. The press of claim 8, wherein N in the TE_(0N) wave mode is equal to
 1. 10. The press of claim 8, wherein the N in the TE_(0N) wave mode is an integer greater than
 1. 11. A microwave preheat press comprising: a first and a second press section defining therebetween a press passage; a compression belt moveable along said press passage contiguous with one of said press sections, the compression belt having an entrance section; the press passage having an initial compression section and a final compression section; a microwave generator; a microwave waveguide applicator structure assembly disposed along the entrance section adjacent the initial compression section, disposed between a fibrous mat and the compression belt, the microwave waveguide applicator structure assembly being configured to direct microwave energy from the generator into the initial compression section; and a choke configured to inhibit arcing between the microwave waveguide assembly and the compression belt.
 12. The press of claim 11, wherein the compression belt is constructed from microwave impervious material.
 13. The press of claim 12, wherein the microwave impervious material is at least one of a stainless steel, molybdenum, polytetrafluoroethylene coated stainless steel, and polytetrafluoroethylene coated molybdenum.
 14. The press of claim 11, wherein the microwave generator forms microwaves having a frequency of at least 95 MHz.
 15. The press of claim 11, wherein the microwave waveguide assembly is configured to produce a TE_(N0) mode waveform.
 16. The press of claim 15, wherein N in the TE_(N0) wave mode is equal to
 1. 17. The press of claim 15, wherein the N in the TE_(N0) wave mode is an integer greater than
 1. 18. The press of claim 11, wherein the microwave waveguide assembly is configured to produce a TE_(0N) mode waveform.
 19. The press of claim 18, wherein N in the TE_(0N) wave mode is equal to
 1. 20. The press of claim 18, wherein the N in the TE_(0N) wave mode is an integer greater than
 1. 